Precise and controllable delivery of laser beams to a desired location is an important technology with respect to telecommunications, military, and other general industrial applications. Beams of light having a low divergence, such as laser beams, play an important role in military and non-military systems, as they can provide a variety of functions, including, but not limited to, infrared countermeasures (“IRCM”), target designation, and communications, such as free-space optical communications. The most common means of obtaining such delivery is by using large, i.e., macroscopic, mechanically controlled mirrors, lenses and gimbals to steer laser beams. While this technology is mature, it is limited by, among other things, the mechanical nature of mirror movement. Furthermore, inertial properties of mechanically driven mirrors limit the speed with which steering can be changed. In addition, gimbaled or rotating mirrors or reflectors may be vulnerable to vibrations and accelerations.
With regard to military applications, such as infrared countermeasures (IRCM), target tracking and designation, and laser communications, at present, further improvement of these applications is hindered by the lack of small, lightweight, low cost, rapid laser beam steering, pointing, and tracking capabilities. The gimbaled and turret mounted laser systems that are currently available tend to be bulky, heavy, expensive and unsuitable for novel battlefield applications. Military applications would also benefit from an ability to emit multiple independently controlled laser beams and from adaptive optics technology. Each of these functions requires, or can benefit from, the ability to point, steer and track the beam. Current technologies have failed to deliver such abilities in accordance with military and defense requirements.
For example, laser beam or free-space optical communications is a particularly useful application of lasers to battlefield situations. The laser's highly directional beam provides the means for rapidly deployed, enormously high bandwidth, and highly secure point-to-point communications links over tactically significant ranges with good relay capability. These laser beam communications capabilities are, however, limited to communications between relatively large, fixed or slow moving objects, because of the slow speed, relatively large weight and significant power consumption of the current turret mounted, gimbaled laser beam steering systems. Employing current technologies, steered laser beams cannot, for instance, be used to provide communications links between rapidly moving or small vehicles such as, but not limited to, small unmanned flight vehicles, individual foot solders, terrestrial vehicles, or other manned or unmanned aircraft. This presents serious short comings in the era of “smart” battlefield and theater of engagement technologies.
Realizing the untapped, battlefield potential of laser beams, the U.S. Defense Advanced Research Agency (“DARPA”) launched the Steered, Agile Beam (“STAB”) initiative in 1999, seeking the development of new beam steering technologies. DARPA specified that the new beam steering technologies should be capable of achieving significant reductions in size, weight, power, and cost over conventional methods. The primary objective of the STAB program was to produce a means to rapidly steer a laser beam over a wide three dimensional angular range while maintaining optical alignment with mobile targets at lengthy target ranges. In particular, the list of potentially useful and desired characteristics of the STAB program include the following specifications and objectives: 1) the ability to achieve a steering field of regard of 180° Azimuth and +/−45° Elevation (i.e., the ability to steer or scan a laser beam better than +/−45°), 2) eye safe operation, 3) rapid acquisition of the intended receiver and maintenance of optical alignment with mobile targets at representative target ranges of from 500 m up to 2 or 3 km, 4) correction for atmospheric degradation (if required), 5) covert optical data communications at extremely high bandwidth or throughput, 6) the ability to operate in the presence of strong daylight, 7) side lobe suppression of better than 30 dB, 8) compatibility with current target designation and IRCM infrastructure, and 9) means for covert target designation. The present invention substantially achieves all of these objectives.
As a result of the STAB initiative, numerous new beam-steering applications have been identified; however, current beam-steering technology still does not exist to support the identified applications by the STAB program. Most current optical beam steering systems continue to be mechanically driven systems—in whole or in part—which are complex, bulky, imprecise and expensive, and require high power to produce desired acceleration of the components thereof. The steering of these systems is relatively slow and imprecise, still often requiring mechanical stabilization, and such systems are still sensitive to vibration and acceleration.
Such shortcomings not only fail to meet the basic battlefield objectives established by DARPA, but further permeate other potential applications that would benefit from rapid, wide-angle agile beam steering. For instance, in the near term, new technologies for beam-steering systems with regard to military aircraft must facilitate self-protection (techniques-based infrared countermeasures or IRCM), targeting, passive and active searching and tracking, and free-space optical communications. Moreover, these systems must accommodate, in the longer term, damage-and-degrade-based infrared countermeasures. The new beam steering technologies must also be “conformal” to the outer skin of a vehicle, such as an aircraft, in order to reduce aerodynamic drag, reduce radar cross section, and minimize the obscuration to adjacent electro-optic systems.
In such cases, the optical beam steering system must deflect or steer an optical beam through relatively large angles, and there is a requirement for both a high speed of deflection and a high degree of precision in positioning the beam. A purely mechanical mirrored beam system can cover a large angular field with high resolution, but the speed or agility of the beam is limited by mechanical inertia. Electro-optical, acousto-optical, and low-inertia mechanical beam deflection systems are capable of high speeds of steering, but have a limited number of resolvable angular positions, typically in the magnitude of +/−1.5-3.0 degrees, and constitute, therefore, small angle or “fine” angle beam steering. As such, there is a need for a rapid, high-angle and precise agile beam steering system for numerous military aircraft applications.
In addition, the ability to rapidly steer multiple beams from a small, light weight package will allow conformal mounting of IRCM systems across all vulnerable points of a military aircraft. Multiple beam steering will also enable deployment of target illumination and designation systems capable of simultaneous engagement of multiple targets. Current technologies have failed to produce a beam steering system able to scan large angles, rapidly and precisely, and with the capability of accommodating more than one beam.
Recent advances in micro component technologies such as liquid crystals, micro electromechanical systems (MEMS) and optical MEMS, resonant cavity photo detectors, micro-diffractive optics, adaptive optics, micro-cavity quantum well lasers, thin film and photonic bandgap materials, for example, offer new opportunities in the development of “chip-scale” Microsystems for steered laser beam applications. However, such technologies are unable to meet all of the objectives of the STAB program and, in particular, are simply unable to deliver rapid, wide-angle and high precision beam steering capabilities.
Beam steering for IRCM applications, therefore, continue to focus on “macro” approaches to resolving the high angle, high speed, high precision dilemma. As such, size and bulk—undesirable features of current macro approaches—continue to plague current beam steering technology. For example, with regard to IRCM technologies, the prior art includes steering mirrors, pointing gimbals and monochromatic electro-optical, beam steering mechanisms. Steering mirrors require output windows many times the size of the system optical entrance pupil to scan over a large field of regard. Unfortunately, the mirror form factor requirements greatly increases the overall size of the sensor package.
By way of demonstration, a particular gimbaled approach for an IRCM device involves use of an imaging system mounted in a dome that is gimbaled to provide a desired pointing angle. The gimbals must point the entire sensor to scan the field-of-regard. Unfortunately, for aircraft applications, this requires a mirror below the platform line, which necessitates a hole in the platform. In addition, the dome and optical assembly is bulky, typically requires considerable volume, and has a radar cross-section which tends to increases the observe-ability of the vehicle.
Other approaches to an IRCM device have been suggested that would utilize existing technologies, such as a ball-turret recessed into the vehicle body. However, the downside of this approach is that, in order to obtain a full field of regard, a large window is required. This approach is further not feasible because the ball-turret must be deeply recessed and positioned within the body of the vehicle. Such an approach would simply utilize too much space within the aircraft vehicle.
Another approach that has been suggested as a conformal package is to implement a rotating prisms concept, which utilizes two prisms that rotate against each other. However, this approach is not desirable because the system is not entirely reflective, and as a result, there is a pointing error among different colors of the spectrum.
There are many other important applications which call for optical beam steering. One of these applications is free space optical communications, which is important to the telecommunications, cable and satellite television industries, as well as the military, as noted above.
From a military perspective, for example, communications networks that form the backbone of tactical communications are most often bulky, heavy, and time consuming to put into operation. Shortfalls in standard military tactical communications include the following:                Frequency allocation is a serious problem.        Bandwidth is too narrow for some traffic needs.        Radio frequency (“RF”) omni-directional emissions allow targeting of defense systems.        Very limited use during periods of radio silence.        RF traffic more easily intercepted by the enemy.        RF signals can be jammed.        Time to set up and relocate RF stations (MSE) takes too long.        Use of wire as an alternative is costly, time consuming and somewhat inflexible.        
Free-space optical communication has a number of advantages over RF communications, not least in the area of security. High performance laser systems have an inherently high level of link transmission security due to the very narrow transmitter beam width. It is necessary to directly interrupt the beam in order to access information, and this is both exceedingly difficult to achieve and easily detectable. For the same reasons, it causes no interference with nearby RF sources. Because lasers operate at a much higher frequency, moreover, they are able to achieve an exponential data throughput improvement. Transferring responsibility for throughput from satellite communication frequencies and into the free-space optical communication world will also free up RF for other military users and for applications that free-space optical communication cannot meet.
Accordingly, there is a need for a beam steering system capable of rapid operation over a wide angular field, and with a high degree of precision. The present invention satisfies this need.
The present invention successfully implements substantially of the aforementioned requirements, including, but not limited to, the DARPA STAB program objectives. The design of the present invention incorporates a high precision small angle steering element or “seeder” utilizing modern technologies such as, but not limited to, electro-optical, acousto-optical, opto-ceramic or piezoelectric actuators and a larger angle steering or amplification feature that is accomplished by spherical reflective devices, e.g., concave mirrors, which amplify the steering angle rendered by the fine-steering element. The novelty of the invention is represented by the amplification of a relative small steering angle, typically less than +/−1.5 degrees to a large steering angle, +/−45 degrees by one or more, but preferably two (2), curved reflectors. Indeed, the present invention can work with most, if not all, of the known small angle “seeder” or steering devices, including such non-mechanical technologies utilizing liquid crystal (LC) or other technologies known to those skilled in the art regarding rapid, small angle, high precision beam steering. The small angle steering can be achieved by any technology with high precision. The invention utilizes the reflection laws of physics and the tremendous speed (3×108 meters per second) at which the light travels, and solves the problems of many other steering schemes, which usually have less than +/−25 degrees of 2-dimensional steering range. To date, no embodiments of the aforementioned concept have been successfully reduced to practice and the prior art has largely failed to successfully accomplish rapid, high precision, large angle beam steering. The present invention accomplishes precise, large angle beam steering in an eloquent fashion.